Video card : ATI radeon XFX 6950 2gb
process :AMD phenom 6X 1100 3.3 ghz
I used to have windows xp so I changed to WIndows 7 Hoem premium
4 GB memory ram
When i play left 4 dead 2 it say lef 4 dead 2e.xe has stopped working outaa nowhere,FPS are great but before i started the game a window poped up saying it couldnt recgonize me video card etc etc i just ignored it and started playing

..... that pop was probably something important. What does dxdiag see the card as? And what version catalyst are you running?

That popup doesn't mean anything. It just says that Steam doesn't uniquely recognize the Graphics card. This Does NOT mean that the game won't run, JUST that steam doesn't UNIQUELY know what the card is.

To the OP:
In addition to the graphics drivers, make sure that ALL other drivers are also up to date. Especially the chipset drivers. Along with DirectX.